A variable displacement hydraulic pump or motor promises the potential for energy savings over throttling valve control. However, the efficiency of variable displacement machines can decrease significantly at low displacement. This can result in poor efficiency for applications operating at partial load for a significant portion of their operating cycle. Exemplary applications include hydraulic hybrid vehicles, hydrostatic transmissions for wind power, and the unique application of compressed air energy storage using a liquid piston. Thus, there is a general need to develop a hydraulic pump or motor with high efficiency across the full displacement range.
Previous variable displacement machines have utilized planar joints that suffer from a trade-off between high mechanical friction and high leakage to maintain hydrodynamic bearings.